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Specifically, low-frequency pure tones that bend around the head with little or no attenuation can only be localized by comparing the time of arrival of a phase of each cycle of the tone at the two ears (the binaural phase-difference cue be- ing a subset of the binaural time cue). However, the phase- difference cue becomes ambiguous for pure tones at high frequencies when successive cycles arrive too quickly for the nervous system to match the arrival of the same cycle at the two ears (Woodworth and Schlosberg, 1954). The exact “frequency of ambiguity” depends on an animal’s head size and the angle of the sound source relative to its midline—it is higher for smaller heads and sound sources closer to the midline.
Pure tones above the frequency of ambiguity, then, must be localized using the binaural intensity-difference cue (Figure 6). Thus, the ability of an animal to use the two binaural cues can be measured by determining the ability to localize pure tones above and below the frequency of ambiguity. Although many of the animals tested so far do use both the binaural time and intensity cues, there are some that have lost the ability to use one or the other cue.
Regarding the binaural time-difference cue, it is probably not surprising that some small animals, such as rats, mice, and some bats, have relinquished use of this cue and rely solely
Figure 6. The ability of vampire bats to localize low- and high- frequency pure tones demonstrates their ability to use both the binaural phase and intensity difference cues. The dip in their per- formance in the midrange is because their auditory system was unable to use the binaural phase cue above 5 kHz, and the binau- ral intensity cue was not physically available until just above 16 kHz. Note that the bat does not use the binaural phase cue from 6.3 to 16 kHz even though it is physically available to them. Modi- fied from Heffner et al. (2015).
Figure 7. Many mammals use both binaural time and intensity dif- ference cues (left side of figure). However, some use only one or the other binaural cue (right side of figure). Of the animals that use only one cue, those with functional interaural distances larger than 400 μs use only the binaural time cue; whereas those with interaural dis- tances smaller than 200 μs use only the binaural intensity cue. Modi- fied from Heffner et al. (2015).
on the binaural intensity-difference cue and pinna cues to adequately localize sound (Figure 7). However, there is no specific functional head size that separates those animals that use the binaural time cue from those that do not. For example, some species that use times cues, such as gerbils, are smaller than some that do not, such as Norway rats. Nor are animals that do not use the binaural time cue necessarily poorer sound-localizers than animals that use both binaural cues. For example, Norway rats with a MAA of about 12° degrees have better acuity than either the chinchilla (17°) or gerbil (27°), both of which use both binaural cues.
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